The present invention generally relates to solenoid valves and more particularly to an improved valve which varies the rate at which hydrocarbons can be removed from an evaporation canister.
An evaporation canister forms a part of an emission control system of an internal combustion engine. When the engine is turned off hydrocarbons in the intake manifold of the engine and the fuel tank are communicated to the evaporation canister which contains a filtering element. Upon starting the engine the free hydrocarbons within the evaporation canister are returned to the intake manifold through a return tube where they are combusted. In order to prevent an excessive amount of hydrocarbons from appearing in the exhaust when the engine is initially started the rate at which the hydrocarbons are permitted to flow from the canister to the intake manifold must be controlled. Prior evaporation canister systems have included orifices or vacuum devices in the return line to control the rate at which the hydrocarbons enter the intake manifold. Vacuum devices are deficient for controlling the flow rate in that they often provide only an on/off control. The present invention provides an improved solenoid which varies the flow rate of hydrocarbons to the intake manifold. Such flow rate may be controlled as a function of engine RPM, temperature, manifold vacuum, etc., in cooperation with an electronic control unit of a known type.
It is a requirement of this type of system to control the hydrocarbon flow rate in an accurate and repeatable manner. To control the accuracy of the flow rate, it is necessary to control the magnitude and direction of the magnetic flux generated upon activation of the solenoid which is lodged within the valve's body.
It is, therefore, an object of the present invention to provide such a valve which embodies an improved magnetic circuit. A further object of the present invention is to provide a valve which exhibits accurate and repeatable cycle-to-cycle performance. Another object of the present invention is to provide a valving element which is guided into seating engagement with a corresponding valve seat.
Accordingly, the invention comprises:
A vacuum valve comprising a housing defining an input port in selective communication with an outlet port. The housing defines a valve seat in communication with the outlet port and includes valve guide means circumferentially positioned about the valve seat. The housing further includes a coil assembly cavity and a coil assembly comprising a bobbin including an axially entending column member and a coil circumferentially wound about the column, the upper bore being axially aligned with the valve guide means. The column includes a partially extending lower bore and a partially extending upper bore. The bobbin further comprises a top member which includes means for receiving a metallic strap. A metallic cylinder is positioned within the lower cavity and extends therefrom. A piston is reciprocally located within the upper bore and positioned for sliding engagement with valve guide means for selectively opening and closing communication between the vacuum inlet and the outlet ports in response to a modulated input signal. A metallic strap is positioned within the receiving means about the coil and includes an arcuately shaped bottom for biasing the metallic cylinder within the lower cavity and defines a preferred magnetic flux path which includes the piston and metallic cylinder.